This invention relates in general to particle separation by density and, more particularly to a dry washer which crushes ore and separates high density particles, such as gold, from a mixture of natural fragments and crushed ore.
Dry washers of several types have long been used in mining gold in desert areas where there is not sufficient water for water sluices and the like. Typical dry washers have a series of inclined troughs and some means for moving dry particulate ore material along the troughs while retaining heavy particles such as gold and magnetite. The final trough generally has a means for trapping the heavy particles, such as Batis cloth with small cross pieces forming riffles in contact with the cloth. Small, high density particles are trapped in the cloth and behind the riffles while larger, low density particles bounce along the trough to an exit. Often, separation is enhanced by mechanisms for vibrating the troughs or for blowing air upwardly through the cloth to blow away small low density particles.
After a suitable quantity of particulate material has been passed through the dry washer, the riffles and cloth are removed and the concentrated fine, high density, material thereon is collected for further processing. Typically, with a small operation, the concentrate can be placed in a conventional gold pan with a small amount of water and panned out to separate gold flakes from particles of other heavy materials such as magnetite ("black sand"). Alternatively, mercury may be mixed with the concentrate to amalgamate with gold and the mercury later removed by distillation or the like. Amalgamation results in a greater recovery of very small particles of "flour" gold.
While conventional dry washers are useful to the hobbyist gold seeker, they are inefficient in that they recover only a relatively small percentage of the gold or other desired high density particles. They also require considerably manual labor in shoveling particulate material such as sand and small gravel into the uppermost trough. A large amount of fine dust is produced and blows around the dry washer, making operating the dry washer uncomfortable. The more efficient dry washers tend to be quite large and heavy, using several sequential troughs and a gasoline engine driven blower to separate light fines from the material. Further, these dry washers are ineffective where the particulate material contains fairly large gravel, since they cannot break up the large particles which may have cracks and crevices containing gold flakes.
A great many other devices have been developed for separating particulate material by size or density for a variety of industrial purposes. For example, vortex chambers such as described by Bielefeldt in U.S. Pat. No. 4,801,310 in which a gas or liquid stream containing particles is rotated rapidly, driving heavy particles to the outside of the chamber by centrifugal force for later recovery are effective with many materials. However, because of their large sizes, complexity, high power requirements and cost such vortex chambers are not useful for small scale gold recovery and the like, especially in isolated areas far from electric power lines.
A number of devices have been developed for pulverizing large particles in a moving gas stream, then collecting the resulting small particles. Typical of these are the fluid current comminuter described by Smith in U.S. Pat. No. 2,624,517 and the moving fluid stream pulverizer of Pipperoux et al. as described in U.S. Pat. No. 2,628,786. Other comminuters use rotating hammers for crushing large particles such as described by Okada et al. in U.S. Pat. No. 3,899,139 and Shepherd in U.S. Pat. No. 3,367,582. While effective, these are very large devices requiring considerable electrical power.
Once large particles have been crushed and high density particles have been concentrated from an ore deposit or the like, gold may be separated by amalgamation with mercury. A number of different amalgamation devices have been designed, such as the amalgamator described by Black in U.S. Pat. No. 681,034. After any gold in a mixture of sand and the like has been amalgamated, the mercury can be separated from the sand with an apparatus such as is described by Wehrel et al. in U.S. Pat. No. 265,898. These devices also tend to be large, cumbersome and require a power source not available in remote areas.
Thus, there is a continuing need for an improved apparatus and method for separating high density particulate material, such as gold, from ore consisting of large and small particles such as sand and rocks. Problems of the prior devices include low recovery efficiency, large size and high power requirements, the inability to effectively crush rocks and the like on a rapid, low power requirement basis, the need for manually shoveling ore into the device and the resulting large clouds of dust around the device. Further, there is a need for improved concentrate recovery and treatment means, including simple and light weight mercury amalgamation means.